| The stress corrosion cracking of pipeline steel and austenitic stainless steel is an important research task in corrosion science field. To prove up the corrosion mechanism of materials in specific media and to search an exact and a convenient inspect method have great significance. The method of metal magnetic memory can diagnose the stress concentration zones. The stress corrosion cracking was almost occurred in the stress concentration zones of the material. And meanwhile, the stress concentration has a close relation of material deformation. X70 pipeline steel is a ferromagnetic material, its magnetic intensity will vary with the stress. The magnetic information can reflect the property of the material. On the other hand, austenitic 304 stainless steel would be induce martensite transformation by cold working. Martensite phase also has the ferromagnetic property. It can reflect the material property by the change of magnetic information. On the base of above appoints, the slow strain rate test was used to study the effect of prestraining on the SCC susceptibility of X70 pipeline steel in simulate soil media. The initiation mechanism of stress corrosion cracking of X70 steel in these environments was studied by electrochemical methods and surface analysis technology. The change rules of magnetic intensity curve during the strain of X70 steel and 304 stainless steel were inspected by magnetic memory instruments. The relationship between SCC susceptibility and magnetic memory effect of these two materials were investigated. The character of magnetic intensity distributing after crack appeared in thematerial was discovered. A series of evaluation criterion of detecting crack by magnetic memory technology were put forward. The main conclusions of the dissertation are below:X70 pipeline steel has a SCC susceptibility in simulate soil environment of high pH medium with 0.5mol ?L'lNa2CO3+1.0 mol ?L'NaHCCb solution and near neutral pH medium of NS4 solution. With the increase of pre-deformation, the plastic loss Is increased in high pH medium with 0.5mol ? L~'Na2CO3+1.0 mol ? L'NaHCCh solution, while the strength loss L has little change. Both plastic loss Is and strength loss L increased with the pre-deformation increasing in NS4 solution.The stress corrosion cracking of X70 pipeline steel in NaHCO3/Na2CO3 system belongs to anodic dissolution mechanism. It is film mechanical rupture. The surface film formed in 0.5mol-L"' Na2CC>3+l mol-L'1 NaHCCh solution on the iron surface in the active-passive electrochemical potential range has a bilayer structure. It is about a few microns thick. The surface film formed at -600mV consists of ferrous carbonates and ferrous hydroxycarbonates. It has a small reaction resistance. It is metastable and poorly protective. The outermost layer is relatively thick, and mainly constituted by coalesced globules. The inner layer is thin and constituted by irregular corrosion products. Many hollows existing in outermost layer, and many microcracks are observed in inner layer. The hollows and microcracks could be active paths for initiation of stress corrosion cracking.During the deformation of X70 pipeline steel, the innerstress increased with the increase of strain and the magnetic intensity of the material surface increased. The magnetic intensity fluctuation amplitude is in agreement with the stress concentration degree. It means that the greater stress concentration, the bigger of magnetic intensity. In different deformation stage, the magnetic distribution on X70 steel surface has different character. Both in corrosive medium and in air, there was a big fluctuation amplitude of magnetic intensity at the crack. The character of the magnetic strength changes suddenly and dramatically can be used to diagnosis the corrosion crack in material.The more SCC susceptibility of X70 steel is in the medium, the little elongation...
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